Rock Climbing Harness with Auto Belay, where to hook on?

I have a fresh new Momentum Speed Black Diamond harness and I'm still
confused, if I were to tie in my rope, I would tie in on the tie in
hoops, but what if I were to Auto Belay? Where do I hook on the swivel
carabiner?

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Straight from the Instruction Manual: This product is a belay device for
the leader or second on a rope.
It has been developed for indoor
wall climbing or for climbing
on well-protected sport routes
where anchors meet the UIAA
standard. It should not be used
for mountaineering or adventure
climbing.

Any modification is a unknown change in the devices individual load bearing rating, and a compromise in the safey of anyone who would be connected to said device at any time after the modification.

I assume you're using the grigri to belay and are looking to extend the position of the Grigri from your harness to produce a sharper angle with your braking hand. I suggest using one locking carabiner attached to your harness and another one attached to your harness, then connect the two with a sewn piece of webbing (or a short length of cord). I would pick a webbing loop no longer than shoulder length, to keep the Grigri from being too far away from you.
I would avoid making any changes to the grigri for the purpose you mention.

The manufacture has clearly defined that this device is designed for 8 mm to 10.5 mm ropes. This type of belay device works on friction, so by adding the extra thickness of a 11 mm rope, increases the friction on the device, thus heating it faster, and possibly taking it to premature failure. I have contacted the manufacture directly for their imput on this subject, and will update you once they respond.

These devices are friction brakes which are designed for descending ropes. Many belay devices can be used as descenders, but there are descenders that are not practical for belaying, since it is too difficult to feed rope through them.
Figure eight
Sometimes just called "eight", this device is most commonly used as a descender, but may also be used as a belay device in the absence of more appropriate equipment.
It is an aluminium (or occasionally steel) "8" shaped device, but comes in several varieties. Its main advantage is efficient heat dissipation. A square eight, used in rescue applications, is better for rappelling than the traditional 8. Because of the "ears" or "wings" on the rescue 8, there is less chance of forming a girth hitch whilst rappelling very quickly.
A figure eight descender
Figure eights allow fast but controlled descent on a rope. They are easy to set up and are effective in dissipating the heat caused by friction but have a tendency to put a twist in the rope. Holding the brake hand off to the side twists the rope, whereas holding the brake hand straight down, parallel to the body, allows a controlled descent without twisting the rope. Because of the many bends it puts into the rope, an 8 descender can wear a rope quicker than a tube style belay/rappel device. Many sport climbers also avoid them because of the extra bulk an 8 puts on the rack. However, many ice climbers prefer to use the 8, because it is much easier to thread with stiff or frozen rope.
Rescue eight
A rescue eight is a variation of a figure eight, with "ears" or "wings" which prevent the rope from "locking up" or creating a girth hitch, thus stranding the rappeller on the rope. Rescue eights are frequently made of steel, rather than aluminium.
Rack
This consists of a 'U' shaped frame, attached to the belayer's harness, into which snap multiple bars that pivot from the other side of the frame. The rope is woven through as many of the bars as are required to provide sufficient friction. This arrangement allows for variations in rope diameter and condition, as well as controlled rate of descent.

These are mechanical friction brake devices used when belaying. They allow careful control of the belay rope. Their main purpose is to allow locking off of the rope with minimal effort. Many types of belay devices exist, and some of these may additionally be used as descenders, for controlled descent on a rope, that is, abseiling or rappeling.
There are passive camming devices and active camming devices. Passive camming devices rely on the brake hand and a carabiner to lock off the rope. Sticht plates and the Air Traffic Controller ATC are examples of passive camming devices.
Active camming devices have a built-in mechanism that locks off the rope without the help of any other pieces of equipment. A GriGri is an example. The offset cam in the GriGri locks off the rope automatically to catch a falling climber, much like a seatbelt in a car locks off to hold a passenger securely. The GriGri fails at around 9 kN of force.
However, a GriGri, with its technology, often makes belayers become less vigilant. The GriGri is not a hands-free belay device. One mistake with the GriGri is reverse threading it. Reverse threading means to thread the GriGri the wrong way around, rendering the camming action useless. However, in a fall, with a reverse threaded GriGri, bending the rope sharply under the GriGri provides more than enough friction to hold a falling climber.
An example of traditional belay is the Body Belay or the Hip Belay, where the rope is wrapped around the body to provide enough friction to catch a climber. This is often used in Alpine climbing, where the routes are easy, and the belay must be fast.
Ice climbers often use a boot belay, where the rope is wrapped around one boot, thus providing friction.